Generally, attacks to private networks or specific servers have features intensively attacking an Internet protocol (IP) address of a target of attack like DDoS. In order to solve the above problem, firewall or virtual private network (VPN) technologies have been generally used. The firewall is used to prevent the corresponding servers or private networks from being attacked due to packets like the DDoS, and the VPN is a technology for controlling an access of only valid users to the private networks. However, the technology may be attacked at all times since the corresponding private networks or servers are exposed to the outside. In order to solve the above problem, the private networks or the servers need to be designed so as not to be exposed to the outside. The description of the virtual private networks is also disclosed in Korean Patent Application No. 10-2004-0001570.